QUESTION: 
How many rockets/missions have landed on Mars and how are we planning to 
prevent littering Mars?

ANSWER from Mike Malin on February 1, 1997:
A very small number of spacecraft have landed on Mars: the former Soviet
Union sent several (I believe the number is 5) during the late 1960's and
early-to-mid-1970's, but all failed.  The United States has only landed two
spacecraft on Mars--Viking Lander 1 and Viking Lander 2 in 1976.  These two
spacecraft were very successful, and sent back pictures and other science
data for as much as three years after landing, but both eventually stopped
working.  The United States presently has a new spacecraft on its way to
land on Mars (called "Pathfinder," it gets there on July 4 of this year),
and a program to send landers to Mars every two years from now on.

Despite being smaller than Earth, Mars is still a VERY big place, and the
landers that have gone to Mars in the past, and those presently planned for
the future, are really very small, so it is unlikely that anyone would come
upon these vehicles on Mars.  There is, perhaps, one lander smaller in size
than a very small car in 7,500,000 square miles, or about one in an area
the size of the United States and Canada.  Do you think this is littering?